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The monitoring of halo hydrocarbons such as R134a in atmosphere has attract great attention due to its strong global warming effect. In this study, on-line atmospheric pressure ionization (API) and electron impact ionization (EI) mass spectroscopy was used to measure R134a at ambient level (ppt, 10-12 mol/mol). A dilution system was developed to test the linearity of API/EI MS. Results showed that the sensitivity of API MS was much higher than that of EI MS. A good linearity could be obtained at the range of 10-75 ppt for API MS. The API MS could directly measure R134a at ambient level without preconcentration devices.